Heterogeneous catalysis is the process of catalyzing a reaction of reagents in one phase, typically a fluid phase, using a catalyst in another phase, typically a solid phase. Effective heterogeneous catalysis requires good contacting between the reagents and the catalyst.
Catalytic converters, which operate through heterogeneous catalysis, have been used by the automobile industry for exhaust aftertreatment since the early 1970s. The first catalytic converters used catalysts supported on pellets beds packed into containers. With these packed beds, it was difficult to keep back pressure within engine tolerances and the pellets tended to degrade over time.
Monolith catalysts have since replaced packed beds in automobile exhaust treatment systems. In a monolith catalyst, the catalytic material is disposed on surfaces in an array of longitudinally oriented channels. A honeycomb is a typical monolith structure. The overall shape of the array gave rise to the term monolith. The structured orientation of the channels results in a reduced pressure drop for a given degree of exhaust-catalyst contacting in comparison with packed beds. The cohesive monolith structure resists degradation better than packed bed catalysts.
Monolith catalyst substrates are generally either ceramic or metallic. Ceramic substrates can be extruded into monolith shapes. Metallic monoliths are formed from metal foils. Typically, at least one of the foils is textured, for example corrugated. The foils can be stacked, folded, or rolled to form a monolith structure. For example, a corrugated foil and a flat foil can be laid one over the other, joined at one end, and rolled to form the monolith structure illustrated by FIG. 1.
In most monolith applications, the surfaces of the monolith channels are entirely coated with catalyst. Catalyst can be coated by dipping the monolith in catalyst slurry (or sol), drawing or pouring catalyst slurry through the monolith channels, or spraying catalyst slurry onto sheets or foils prior to assembling them into the monolith.
In some applications, it is preferred or necessary to coat only a fraction of the monolith channels. U.S. Pat. No. 7,240,483 describes a pre-combustion catalyst that preferably has catalyst coated on only a fraction of its channels. The pre-combustion catalyst is designed to promote low temperature start-up of a fuel reformer and to increase fuel reforming efficiency. The pre-combustion catalyst operates by combusting a first portion of injected fuel while vaporizing a remaining portion of the fuel. Heat released by combustion provides the heat of vaporization. Vaporized fuel and exhaust leaving the pre-combustor are allowed to mix prior to entering the fuel reformer. Vaporizing the fuel prior to mixing makes mixing more effective. The improvement in uniformity promotes efficient reformate production and reduces hot spots in the fuel reformer.
Coating only a fraction of the pre-combustor channels ameliorates the occurrence of hot spots within the pre-combustor, prevents the pre-combustor from overheating, and prevents more than the desired fuel fraction from combusting in the pre-combustor. The pre-combustor is required to operate under a wide range of conditions and with poorly mixed feeds. Heat generated in coated channels is spread by heat transfer to fuel and exhaust passing through adjacent uncoated channels. Overall fuel conversion and heat generation are generally limited by having only the fraction of fuel entering the coated channels contact the catalyst. The uncoated channels are fluidly isolated from the coated channels; the fraction of fuel entering those channels is vaporized but does not contact the catalyst and will not combust absent very high temperatures.
In the prior art, pre-combustor catalysts with only a fraction of the channels coated have been prepared by spray-coating metal sheets on one side only. Folding the coated face onto itself, or laying two sheets together with the coated sides facing each other, followed by rolling, provides a suitable structure. Spray-coating, however, is difficult to carry out on a production scale.
Processes for coating alternate channels of diesel particulate filters do not provide attractive alternatives for making pre-combustor catalysts. On a commercial scale, these processes involve precast tools that pick out alternate channels of ceramic substrates. Pre-combustor catalysts are preferably made from metal substrates. Metal provides advantages in terms of allowing for structurally sound thin monolith walls having low thermal inertia and high thermal conductivity. Unlike ceramic monoliths, metal monoliths are not made by extrusion and the exact location of the channels varies significantly from sample to sample. Picking out alternate channels of metal monoliths and directing catalyst into just those channels would be difficult.
The problem addressed by the inventors was to develop a process suitable for preparing partially coated metallic-substrate monolith catalysts that better lends itself to commercial production than the prior art spray-coating process.